The present invention relates generally to optical filters, and more particularly to single or multiple band high reflection optical filters having increased laser damage threshold through reduction of the standing wave field profiles at the wavelengths of the invasive laser wavelengths to be reflected.
A multi-layer stack high reflection optical filter usually includes a substrate having a layered optical film thereon facing incident potentially damaging radiation. The film normally has alternate high and low refractive index dielectric layers having optical thicknesses of a quarter-wavelength of the radiation to be reflected. Each layer interface presents optical, thermal and mechanical mismatches highly susceptible to laser damage.
Efforts to increase the laser damage threshold probability of multi-layer stack high reflection filters have included changing layer thicknesses to move maximum flux densities in the standing wave field profile away from layer interfaces, such as by changing the optical thicknesses of a layer pair added to the exposed side of the filter, by modifying the optical thicknesses of up to three layer pairs added to the exposed side of the filter, and by adding a half-wave thick overcoat layer of the low refractive index material to the exposed side.
Efforts to increase laser damage threshold of filters having graded refractive index profiles have included mixing two materials to relieve film stress, using porous antireflective surfaces and coatings (graded refractive index surface layer), and adding two graded refractive index layers of half-wave optical thicknesses to a quarter-wave optical thickness multi-layer stack.
In improved optical filters of this invention, laser damage threshold is increased at wavelengths coincident with laser radiation to be reflected by reducing standing wave field profiles within optical films comprising the filters. Filters structured according to the invention comprise continuously varying sinusoidal refractive index profiles with selected number of periods and selected phase at an interface where incident laser light impinges the filter in single band configurations, and superimposed multiple sinusoidal refractive index profiles with selected phase alignment of superimposed profiles at the interface where laser light impinges the filter in multi-band configurations.
Increased laser damage threshold of optical filters provided by the invention has application to laser survivability of high reflection optical filters operating in the ultraviolet, visible, infrared and microwave including dichroic beam splitters, dielectric laser mirror coatings, dielectric hot/cold mirrors and laser protection passband optical filters.
It is therefore a principal object of the invention to provide an improved optical filter.
It is a further object of the invention to provide an optical filter which may be configured for operation in the ultraviolet, visible, infrared or microwave regions of the spectrum.
It is a further object of the invention to provide an optical filter wherein selective alignment is made of the phase of sinusoidal refractive index profiles of an optical film at an interface where potentially damaging laser light is incident.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.